Inexpensive light modulators that have high contrast and wide optical bandwidths have been used in optical wavelength-division-multiplexing networks. A modulation device particularly well suited for the above application is a surface normal mechanically-active an-reflection switch (MARS) modulator. This device may be described as having a variable air gap defined by two layers of material. Typically, surface normal MARS modulators operate by changing the amount of light reflected in the surface normal direction, i.e., the direction normal to the substrate surface. This may be achieved by varying the variable air gap, which alters the optical properties of the device.
One such MARS modulator has been described by Aratani et al. in “Process and Design Considerations for Surface Micromachined Beams for a Tuneable Interferometer Array in Silicon,” Proc. IEEE Microelectromech. Workshop, Ft. Laud., Fla., Feb. 7-10, 1993 at 230-35. This article, and all other articles referenced in this specification are herein incorporated by reference in their entirety. Aratani's modulator is described as having a diaphragm mirror consisting of a polysilicon/silicon nitride multilayer supported by thin beams over a substrate, also partially mirrored by a polysilicon/silicon oxide multilayer. As a voltage is applied between the membrane and the substrate, the membrane is pulled toward the substrate. While a large change in reflectivity is supposedly achieved, the optical bandwidth of the optical resonator based modulator is limited. The contrast ratio of such a device falls off sharply as the wavelength of the incident light varies from the resonant wavelength of the device.
A second MARS modulator was described by Solgaard et al. in “Deformable Grating Optical Modulator,” Optics Lett. 17(9) 688-90 (1992). This modulator was described as having a reflection phase grating of silicon nitride beams that is coated with metal and suspended over a substrate coated with metal. An air gap separates the grating and substrate. The deformable grating optical modulator described in Solgaard et al. does not achieve a low reflectivity state. Rather, it switches to a diffracting state. In the diffracting state, incident light is scattered into higher-order diffraction modes of the grating, so that the amount of light reflected into the zero order (surface-normal) mode is minimized. Such diffraction may be an undesirable aspect of the deformable grating optical modulator. If the numerical aperture of the incoming fiber or detection system is large enough to pick up the higher order diffraction modes, a degradation in contrast will result. Further, if this device is implemented in a system using arrays of optical beams or fibers, a significant crosstalk may be introduced.
U.S. Pat. No. 5,500,761 to Goossen (the Goossen '761 patent) describes a non-contacting MARS modulator which provides high contrast modulation for optical signals over broader range of wavelengths. More particularly, the modulator of the Goossen '761 patent describes a device comprising a membrane containing a layer whose refractive index is nearly the square root of that of the substrate, and whose thickness is a quarter of a wavelength of the light (as measured in the layer). Subsequent U.S. Pat. Nos. 5,589,974 and 5,654,819, both to Goossen (the Goossen '974 and '819 patents) describe further embodiments of the device described in the Goossen '761 patent. The Goossen '761, '974, and '819 patents are incorporated herein by reference in their entirety. In one embodiment described in the Goossen '819 patent, the membrane consists of three or more layers suspended over a substrate by support arms. There is a specific relationship between the refractive indices of the membrane layers and the refractive index of the substrate, and the membrane layers have specific thickness. The gap between the membrane and the substrate in the biased state is λ/4 and the air gap 20 between the unbiased membrane 15 and the substrate 10 ranges from about 0.65λ to about 0.7λ (Column 5, Line 40 to Column 6, Line 5).
All the MARS devices so far described are concerned with operation with substantially surface-normal (perpendicular) light (as they were primarily concerned with fiber optical modulation). For free space communications, there is a need for a MARS modulator that can be optimized to operate over a wide range of angles of incidence at a specific frequency. Solid state semiconductor modulations, such as those made from indium phosphide, have been suggested for use in free space communications. The main drawback of such devices is their expense, which can be up as much as 250 times the cost of standard MARS optical modulators.